Most conventional magnetic recording media are of the coated type which are produced by dispersing particles of magnetic oxides or ferromagnetic alloys, such as .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, a Berthollide compound of .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4 or CrO.sub.2, in an organic binder, such as a vinyl chloride/vinyl acetate copolymer, a styrene/butadiene copolymer, an epoxy resin or a polyurethane resin, applying the resulting coating composition onto a non-magnetic base and drying the coating.
However, with the recent increasing demand for higher density recording, researchers' attention has been drawn to magnetic recording media of the thin metal film type that comprise a thin ferromagnetic metal film as a magnetic recording layer, the layer being formed by a vapor deposition technique such as vacuum deposition, sputtering or ion plating, or a plating technique such as electroplating or electroless plating. Various efforts are being made to use such recording medium on a commercial scale.
Most magnetic recording media of the coated type use a metal oxide with low saturation magnetization as a magnetic material, so attempts to achieve high density recording using a thinner magnetic recording medium results in a decreased signal output. However, with magnetic recording media of the thin metal film type, a very thin magnetic recording layer can be formed using a ferromagnetic metal having a higher saturation magnetization than with a magnetic oxide without using a non-magnetic material such as binder, and such thinness is advantageous to provide good electro-to-magnetic conversion characteristics. However, such thin metal film type magnetic recording media have their own problems:
(1) high friction against a magnetic head, guide poles or other transport means when run to record, reproduce or erase magnetic signals, i.e., the same wear easily;
(2) easily attacked by corrosive environments; and
(3) the magnetic recording layer thereof may be damaged by impact during handling.
Some attempts have been made to solve these problems by forming a protective layer on a magnetic recording media of the thin metal film type. One such proposal is described in Japanese Patent Application (OPI) No. 75001/75 (the symbol OPI as used herein means as unexamined published Japanese Patent Application) wherein a thin lubricant layer is formed on the thin metal film. According to this proposal, the coefficient of friction between the magnetic head or guide poles and the thin metal film is reduced to provide a tape that runs consistently and which is unlikely to be abraded. However, these advantages are quickly lost if the tape is used repeatedly.
Another method is described in Japanese Patent Applications (OPI) Nos: 39708/78 and 40505/78 wherein a lubricant protective layer made of a metal or metal oxide is formed on the thin metal film; however, even in this case the effect of the protective layer does not last long and, as the tape is repeatedly used, the friction coefficient increases rapidly or the thin magnetic metal film breaks.
Still another method is described in Japanese Patent Application (OPI) No. 155010/79 wherein an overcoat of a high molecular weight film is formed on the metal film; however, if the overcoat is made of a vinylidene chloride/acrylic ester copolymer or some other known high-molecular weight substance, the resulting film thickness is at least about 0.2 .mu.m which causes spacing loss which, in turn, reads to reduced output in high density recording.
To achieve high density recording, most thin magnetic metal films are supported on a very smooth base, but no matter how smooth the base surface is, none of the lubricating methods described above can provide a magnetic recording medium having good running property especially in very humid atmospheres, or high wear resistance.